I was in Reno for the first time a week ago and witnessed a most interesting communal building of nests by cliff swallows. The Truckee River runs right through the middle of downtown Reno and the swallows are building their nests beneath the bridges crossing the river. They were flying all about as they normally do, but what they were doing on the banks of the river, I had never seen before. They were all fluttering, and at first I thought these were baby birds and their parents were feeding them; it was that kind of FLUTTERING. However, it didn't seem like anyone was getting fed. Then I thought perhaps it was a mating ritual; but no mating was going on. Finally I realized that they were doing some sort of MUD DANCE and working themselves up, grabbing mud in their beaks and then flying up to continue the building of their mud nests. It was quite amazing and like nothing I had seen before. Here is a link to the event:http://www.bing.com/videos/search?q=Cliff+swallows+building+their+nests&view=detail&mid=7CC872D78A7348B798F37CC872D78A7348B798F3&first=0&FORM=NVPFVR